Hill, who will soon retire after a 35-year career, believes William will be with Kate every step of the way. The couple is "a very strong team," she says, adding that "Prince William will be doing the right stuff on the day."

Hill – who is a pediatric and obstetric physiotherapist and author of a book, Christine Hill's Pregnancy Guide – has some tough-love advice for expectant mothers.

"No one prepares you for the true ghastliness of birth," she says. "There's a sort of whimsical attitude that it will all be lovely and that everyone can have a natural birth. But that's not true."

And her tips for dads-to-be like William? "I tell the fathers that the girl they hitched up with has gone for now – that she is fragile and vulnerable, and will be through pregnancy, labor and the first few weeks," Hill tells the paper.

Christine Hill

Andrew Crowley

And despite Kate being very sporty, Hill prefers for women to lie "around during pregnancy," she says. "They shouldn't be jogging."

Kate and William saw Hill during private sessions at a residence by the Thames in west London. Sophia Money-Coutts, an editor at the upscale Tatler magazine, tells PEOPLE, "The lessons take place at a house in Chiswick, and you have your classes looking out over the river."

But Kate most likely didn't rely only on Hill for advice. "There are lots of friends who have little ones or who are expecting," Money-Coutts says. One of those recent mums is Marina Fogle and her sister Chiara Hunt, who help run The Bump Class in London.

Fogle, who also used Hill with her TV-presenter husband Ben, won't confirm if Kate has been to them but recently said that she will be "very well looked after."

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